In producing high-quality carbon materials such as needle coke, carbon fiber or the like from coal tar, quinoline insoluble components (hereinafter referred to as "QI components") contained as impurities in the coal tar must be removed to the utmost extent. The QI components in coal tar are carbonaceous materials in the form of fine particles 0.3 .mu.m or less in particle size (such QI components are generally called "primary QI components"). When coal tar contains a large amount of primary QI components, the primary QI components tend to adhere to the surface of mesophase bodies (spherulites) generally called "secondary QI components" during the heat treatment of the coal tar. The adhesion of primary QI components is presumed to inhibit the coalescence of the spherulites and to hinder their normal growth.
For this reason, the removal of primary QI components from coal tar has been recognized as important. For example, a method has been practiced in which oil of such property that the oil and coal tar are hardly miscible in each other, e.g. petroleum-type light oil, is added to coal tar to aggregate the primary QI components of the coal tar into particles of increased particle size and the mixture is left to stand to separate the enlarged primary QI components by sedimentation (Japanese Unexamined Patent Publication No.28501/1977). This method requires the distillation of the supernatant liquid after removing the primary QI components in order to get tar and/or pitch as a useful component and recovery the oil initially added by distillation. The distillation of the liquid requires a great amount of thermal energy and also results in low yields of the useful component. Further, the oil recovered by the distillation is a mixture of the petroleum-type oil added and coal-type oil derived from coal tar and thus has a limited value unless further treated. Since the precipitate phase separated by standing contains a large amount of petroleum-type oil, the oil as added would be recovered at a low ratio if the precipitate is not subjected to a treatment for the recovery of oil. The recovery treatment requires equipments such as a distillation column, tanks, etc. Moreover, this method involves the use of a large-size tank for storing petroleum-type light or middle oil to be used and related installations, consequently demanding a wide space for arrangement of the equipment.